Asexual Reproduction
The Basics
- → Asexual reproduction does not involve sex cells or fertilisation.
- → Only one parent is required.
- → This means there is no fusion of gametes and no mixing of genetic information.
- → As a result, the offspring are genetically identical to the parent and to each other. These are called clones.
- → Definition: A process resulting in genetically identical offspring from one parent.
- → Asexual reproduction is more common in plants than in animals.
Examples of Asexual Reproduction
Bacteria
Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission.
Space for Diagram: Binary Fission in Bacteria
The process is as follows:
- The parent bacterial cell's contents, including the circular chromosome, double.
- The genetic material (DNA) is replicated.
- The cell divides into two.
- The result is two genetically identical daughter cells.
Fungi
Some fungi, like yeast, reproduce asexually by budding. A new cell grows out of the surface of the parent cell, receives a copy of the nucleus, and then detaches.
Plants
Some plants grow new plants from runners (e.g., strawberry plants) or tubers (e.g., potatoes). These are stems that grow new, genetically identical plants.
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Sexual Reproduction
Core Concepts
- → Definition: A process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other.
- → Because there are two parents, the offspring contain a mixture of their parents' genes.
Gametes and Fertilisation
- Gametes in animals: Sperm cells (male) and egg cells (female).
- Gametes in plants: Pollen (contains the male gamete) and egg cells (female).
- Fertilisation is defined as the fusion of gamete nuclei.
- A zygote is the new cell formed by fertilisation. It will then undergo cell division and develop into an embryo.
- This mixing of genetic information leads to variation in the offspring.
Comparing Reproduction Types
| Feature | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Number of Parents | One | Two |
| Gametes | Not involved | Male and female gametes fuse |
| Genetic Information | Offspring are genetically identical (clones) | Offspring have a mixture of genes, causing variation |
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Sexual Reproduction in Plants
Structure of an Insect-Pollinated Flower
Flowers are the reproductive organs of plants.
Space for Diagram: Insect-Pollinated Flower Structure
| Part | Function |
|---|---|
| Sepals | Protect the unopened flower. |
| Petals | Often brightly coloured to attract insects. |
| Nectaries | Secrete nectar to attract insects. |
| Stamen | The male part of the flower, comprising the anther and filament. |
| Anther | Produces the male gametes, which are inside pollen grains. |
| Filament | The stalk that supports the anther. |
| Carpel | The female part of the flower, comprising the stigma, style, and ovary. |
| Stigma | The top part of the carpel that collects pollen grains. |
| Style | Connects the stigma to the ovary. |
| Ovary | Contains the female gametes inside ovules. |
Pollination
Definition: Pollination is the transfer of pollen from the male part of the plant (anther of stamen) to the female part of the plant (stigma).
- Self-pollination: The transfer of pollen from an anther to the stigma of the same flower, or a different flower on the same plant. This leads to less genetic variation.
- Cross-pollination: The transfer of pollen from an anther of one plant to the stigma of a flower on a different plant of the same species. This leads to more genetic variation.
Space for Diagram: Wind-Pollinated Flower Structure
Features of Insect vs. Wind-Pollinated Flowers
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | Large and brightly coloured | Small, dull (often green or brown) |
| Scent & Nectar | Usually scented with nectar | No scent or nectar |
| Pollen Grains | Small quantity, large and sticky/spiky | Large quantity, small, light and smooth |
| Anthers | Stiff, firmly attached, positioned to brush against insects | Hang loosely on long filaments to release pollen easily |
| Stigma | Sticky, located inside the flower to brush against insects | Feathery, hangs outside the flower to catch drifting pollen |
Fertilisation, Seed & Fruit Formation
- When a pollen grain lands on the stigma of a plant of the correct species, a pollen tube begins to grow.
- The tube grows down through the style, guided by chemicals secreted by the ovule.
- The nucleus of the male gamete passes down the pollen tube.
- Fertilisation occurs when the male gamete nucleus fuses with the female gamete nucleus inside the ovule, forming a zygote.
- The zygote divides by mitosis to form an embryo.
After fertilisation:
- The zygote develops into an embryo.
- The ovule develops into a seed.
- The ovary develops into a fruit, which protects the seed and aids in dispersal.
Seed Structure & Germination
Space for Diagram: Structure of a Seed
A seed contains:
- Embryo: Consists of the plumule (embryo shoot) and radicle (embryo root) that will grow into the adult plant.
- Food store (cotyledons): A source of energy (starch) for the embryo.
- Seed coat (testa): A tough outer layer that protects the embryo.
Germination is the start of growth in the seed. For this to happen, it needs specific conditions:
💧 Water
Needed to activate enzymes to break down the food reserves.
💨 Oxygen
Needed for respiration, which transfers the energy for growth.
🌡️ Warmth
Needed for enzymes to work efficiently (at their optimum temperature).
The developing embryo uses its food stores for energy (via respiration) to grow. Once the young plant has grown its first leaves, it can begin to produce its own food by photosynthesis.
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Sexual Reproduction in Humans
Reproductive Systems
Male Reproductive System
Space for Diagram: Male Reproductive System
- Testes: Produce sperm (male gametes). Held in the scrotum outside the body to keep them at a temperature slightly lower than body temperature.
- Sperm ducts: Carry sperm from the testes to the urethra.
- Prostate gland: Secretes fluids for sperm to swim in, forming semen.
- Urethra: A tube that carries both semen (during ejaculation) and urine out of the body.
- Penis: Places sperm into the vagina during sexual intercourse.
Female Reproductive System
Space for Diagram: Female Reproductive System
- Ovaries: Contain eggs (female gametes).
- Oviducts (Fallopian tubes): Connect the ovaries to the uterus. Lined with cilia (tiny hairs) to sweep the egg along. This is where fertilisation happens.
- Uterus (womb): Has a soft lining where the embryo grows and develops.
- Cervix: A ring of muscle at the entrance to the uterus.
- Vagina: A muscular tube that receives sperm during sexual intercourse.
Gamete Comparison: Sperm vs. Egg
| Feature | Sperm | Egg |
|---|---|---|
| Size | Very small | Large |
| Cytoplasm | Very little | Large amount, contains a food store |
| Mobility | Has a tail for swimming (motile) | Cannot move on its own (non-motile) |
| Numbers | Produced in millions | One released each month |
Fertilisation and Development
- During sexual intercourse, semen is ejaculated from the penis into the vagina.
- Sperm cells swim through the cervix, uterus, and into the oviducts.
- If an egg is present in an oviduct, fertilisation can occur (the fusion of a sperm nucleus and an egg nucleus).
- The fertilised egg, now a zygote, divides to form an embryo.
- The embryo travels to the uterus and embeds itself in the uterus wall. This is called implantation.
Pregnancy: Supporting the Embryo
- Amniotic sac & amniotic fluid: The amniotic sac is a membrane bag that surrounds the embryo and secretes amniotic fluid. This fluid protects the fetus from shocks.
- Placenta & umbilical cord: The placenta develops from the uterine wall and allows for the exchange of substances between the mother's blood and the fetus's blood.
- From mother to fetus: Oxygen, nutrients.
- From fetus to mother: Carbon dioxide, waste products.
- The mother's blood and the fetal blood do not mix. Substances are exchanged by diffusion. The placenta has a large surface area and thin wall for efficient diffusion.
- Harmful substances like nicotine, alcohol, and viruses can also cross the placenta.
- The umbilical cord connects the fetus to the placenta.
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Sexual Hormones in Humans
Puberty: The Changes of Adolescence
Puberty usually starts between the ages of 10-14 in girls and 12-16 in boys.
Role of Testosterone in Males
During puberty, the testes start producing the hormone testosterone, which causes secondary sexual characteristics to develop:
- Voice breaks (deepens)
- Growth of hair on face, under arms and in pubic area
- Body becomes more muscular
- Testes start to produce sperm
Role of Oestrogen in Females
During puberty, the ovaries start producing the hormone oestrogen, which causes secondary sexual characteristics to develop:
- Growth of hair under arms and in pubic area
- Breasts develop
- Hips get wider
- The menstrual cycle begins
The Menstrual Cycle
The menstrual cycle is a monthly cycle of changes in the female reproductive system.
Space for Diagram: The Menstrual Cycle
The cycle involves four main stages:
- Stage 1: Menstruation. The lining of the uterus breaks down and is released. This is known as having a period.
- Stage 2: Uterus lining builds up. The uterus lining grows again into a thick, spongy layer full of blood vessels, ready to receive a fertilised egg.
- Stage 3: Ovulation. An egg is released from one of the ovaries (at approximately day 14).
- Stage 4: Lining is maintained. The wall is maintained for about 14 days. If no fertilised egg has landed on the uterus wall by the end of the cycle, the lining starts to break down and the cycle starts over again.
Note: You do not need to know any details of the hormones involved in the menstrual cycle (FSH, LH, oestrogen, and progesterone).
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Sexually Transmitted Infections (STIs)
STIs & HIV/AIDS
- Unprotected sexual intercourse can lead to the transfer of pathogens via the exchange of body fluids.
- Infections passed on this way are known as sexually transmitted infections (STIs).
How HIV Affects the Immune System
- HIV (Human Immunodeficiency Virus) is an example of an STI. It is a viral pathogen.
- The virus infects the body's immune system and decreases the body's ability to fight infection.
- This eventually leads to AIDS (Acquired Immunodeficiency Syndrome).
- HIV can be spread by infected body fluids, e.g.
- Sharing needles with an infected person.
- Blood transfusions with infected blood.
- From mother to fetus across the placenta during pregnancy.
- Mother to baby via breastfeeding.
- Having unprotected sexual intercourse with an infected person.
Controlling the Spread of STIs
The spread of STIs such as HIV is best controlled by:
- Not having unprotected sex, but making sure to always use a condom.
- Getting tested if unprotected sex or sex with multiple partners has occurred.
- Raising awareness through education programmes.
- Never sharing needles and ensuring medical equipment is sterile.
- Screening for HIV during pregnancy.